Recently, various multiplexing techniques have been implemented for radio communication systems to more effectively utilize a limited frequency band. For example, a multiple input multiple output (MIMO) radio communication system is known in which multiple communication data series using the same frequency band are simultaneously transmitted through multiple transmitting antennas, and in which the multiple communication data series are received through multiple receiving antennas and then are separated into individual communication data series (for example, Patent Document 1).
In such a radio communication system, the distance between the antennas is generally set to be much longer (for example, not less than 4λ) than the wavelength of the frequency in use in order to improve separation of communication data series in a receiving device by producing diversity effect from the arranged positions of the antennas.